Hollow gas-filled glass or metal spheres or target pellets are known (see Wang and Elleman, U.S. Pat. No. 4,344,787, "Method and Apparatus for Producing Gas-Filled Hollow Spheres"). Molten material is forced from a nozzle by a piston while a high-pressure gas fills the center of the liquid column as it emerges from the pipe. The molten material emerges into a gas-filled environment and breaks into hollow spheres.
Nuckolls et al., U.S. Pat. No. 4,376,752, "Foam Encapsulated Targets" uses a similar process to fabricate glass or metal target pellets containing thermonuclear fuel. However, vibrations are applied to the extruded tube to assist its breakup into uniform hollow bodies. Teitel et al., U.S. Pat. No. 4,432,933 discloses the use of not only glass and metal but also ceramic, carbon and plastic in manufacturing hollow microspheres for application in thermonuclear fusion reactors.